Auditory lateralization in monkeys: An examination of two cues serving directional hearing

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Abstract
In assigning binaural ongoing time differences (phase) as the cue for localization of low frequencies and binaural intensity differences as the cue for localization of high frequencies, the duplex theory has successfully accounted for human directional hearing of tones. Sensitivity of monkeys to these cues was examined in 2 experiments. The dependencies on frequency of interaural intensity difference thresholds (lateralization experiment I) and time difference thresholds (lateralization experiment II) were determined behaviorally on 3 monkeys (M. nemestrina). The range of frequencies was from 125 Hz to 8 kHz in experiment I and from 250 Hz to 2 kHz in experiment II. The duplex theory is applicable to monkeys. Monkeys are less sensitive than man to both binaural cues. The shortest time disparity monkeys discriminate is 42 .mu.s at 1.5 kHz and the smallest intensity difference is 3.5 dB at 500 Hz. Monkeys appear to utilize time disparity cues through higher frequencies than man. Inaccurate localization by monkeys at high frequencies reflects decreasing sensitivity to interaural intensity cues.

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